Addition of organoboron compounds

The catalytic H—B addition of organoboron compounds to unsaturated molecules is synthetically interesting in main-group chemistry, as a wide range of functional groups can be transformed from the selective new C—B bond [1]. Although considerable energy has been expended to use rhodium] I) complexes to catalyze the... 

Palladium-catalyzed hydroboration, haloboration, and similar reactions involving addition of organoboron compounds to alkynes have been reviewed.Alkylborane addition to alkynes followed by intra- or intermolecular cross-coupling with a vinyl halide or triflates according to the Suzuki-Miyaura protocol constitutes a very powerful synthetic route for the generation of two carbon-carbon bonds in cascade. This subject and its applications in synthesis have been reviewed. ... 

The overall mechanism is closely related to that of the other cross-coupling methods. The aryl halide or triflate reacts with the Pd(0) catalyst by oxidative addition. The organoboron compound serves as the source of the second organic group by transmetala-tion. The disubstituted Pd(II) intermediate then undergoes reductive elimination. It appears that either the oxidative addition or the transmetalation can be rate-determining, depending on reaction conditions.134 With boronic acids as reactants, base catalysis is normally required and is believed to involve the formation of the more reactive boronate anion.135... 

The reaction is called hydroboration and is a versatile synthesis of organoboron compounds. One example is the addition of diborane, B2H6, to ethene. Di-borane behaves as though it is in equilibrium with BH3 (B2H6 <= 2BH3), and addition proceeds in three stages ... 

The covalent B-G bond of organoboron compounds shows little reactivity toward representative electrophiles, but the presence of an empty low-lying orbital facilitates transmetallation to other metals. The transmetallation to zinc709 and other metal reagents has been extensively studied for carbon-carbon bond formation via addition and coupling reactions of trialkylboranes and alkylboronic acids. 

The metal-catalyzed addition reaction of organoboron compounds have not yet been well developed, but die reaction of NaBPh4 or arylboronic acids with enones in the presence of Pd(OAc)2 and NaOAc or SbCL, was recently reported by Uemura and his coworkers.2 The reaction was proposed to proceed through the oxidative addition of the C-B bond to the Pd(0) species however, another probable process, the transmetalation to transition metals, may allow a similar catalytic transformation by the use of organoboronic acids. We report here the 1,4-addition reaction of organoboronic acids to oc, J-unsaturated ketones or esters and 1,2-addition to aldehydes or imines catalyzed by a (acac)Rh(CHj=CH2)2/phosphine complex, which may involve the B-Rh transmetalation as the key step. 

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